I got my betta fish, Terrence Aki (Teri-yaki) about a month ago and from day 1 he was a very active fish but within the past week hes been acting crazy! He's swimming eratically up and down and sideways and seems to be sort of bending to the right. I have also noticed that his tail seems to be thinning out! What could be the explanation for this ?!?!?!???? Pls help me!

Housing
What size is your tank? aprox. 2 litres
What temperature is your tank? room temp. 22 C
Does your tank have a filter? No
Does your tank have an air stone or other type of aeration? No
Is your tank heated? No
What tank mates does your betta fish live with? None, just a plastic plant

Food
What type of food do you feed your betta fish? Freeze dried worms
How often do you feed your betta fish? once a day 3-5 worms

Maintenance
How often do you perform a water change? every 10 days
What percentage of the water do you change when you perform a water change? 75%
What type of additives do you add to the water when you perform a water change? Betta Plus bowl conditioner

Water Parameters:
Have you tested your water? If so, what are the following parameters? No

Symptoms and Treatment
How has your betta fish's appearance changed? tail has slightly thinned out
How has your betta fish's behavior changed? Swims eratically
When did you start noticing the symptoms? week ago
Have you started treating your fish? If so, how? No
Does your fish have any history of being ill? No
How old is your fish (approximately)? Unknown

Your aquarium is roughly 0.5 US gallons, there's a lot of discussion on the forums as to the minimum tank size for a betta fish and many say that 2.5 gallons (about 10 litres) is the absolute minimum; some members have successfully kept their betta fish in 1.5 gallons (about 6 litres) and with regular water changes the fish has done well. To be honest, the bettas home seems a little on the small side to me.

Your water changing schedule may need to be looked at as well, I'm afraid. Aquariums of 2.5 gallons or less (especially those without a filter) I believe need at least three water changes a week, or two water changes: one a 50% water change and the other a 100% water change (with thorough cleaning throughout, meaning substrate, ornaments, wiping down the inside of the aquarium etc).

The reason I mentioned your tank size and water changing schedule is because these two things can make a betta fish act strangely due to toxic substances in the water building up. These toxins are fish poo and pee (ew!) and need to be removed on a regular basis; in a smaller tank these things can build up very quickly and cause medical issues. Adding a filter helps reduce maintenance required, but regular water changes will always be a priority.

Perhaps surprisingly, a larger aquarium is actually easier to take care of than a smaller one. It also gives you more viewing space and allows for more decorations. Decorations give bettas mental stimulation and make them very happy, it's also good to include hiding places like caves in case they get spooked.

As well as the above, bettas are tropical fish and ideally should be kept between 78F and 81F (25C to 27C), below 75C or so you might find your betta starts to become lethargic. Heaters are usually required to keep these fish alive long-term, though in some areas the air temperature is enough. It looks like your tank is quite chilly for your tropical friend.

Frozen blood worms are a fantastic treat for betta fish but can cause digestive issues if fed as a regular food in large amounts. I do suggest also feeding your betta some form of flake or pellet food (1 - 2 pellets/flakes twice a day is a good routine). This will help your betta's digestion and prevent bloating.

The things I listed could all be reasons as to why your fish is acting strangely; I'm sorry you're having problems and I hope other members can help you as well. Good luck!